1. Field of the Invention
This invention generally relates to a bearing unit having a spherical sliding surface and its manufacturing method, and, in particular, to a greaseless spherically sliding bearing unit suitable for use in various link mechanisms in transmission and steering systems of automobiles or the like.
2. Description of the Prior Art
A spherically sliding bearing unit is well known in the at art and its typical example is illustrated in the Japanese Patent Post-examination Pub. No. 51-42569 and shown here in FIGS. 8 through 9.
As shown in FIG. 8, a spherical surface sliding bearing unit typically includes an inner member 50 having a spherical outer peripheral surface, an outer member 51 having a spherical inner peripheral surface opposite to and spaced apart from the spherical outer peripheral surface of the inner member 50 and a liner or self-lubricating thin plate 52 interposed between the inner and outer members 50 and 51. The liner 52 is typically comprised of a low friction high polymer material such as fluorine family resin and fixedly attached to the outer member 51 so as to provide a greaseless sliding contact at an interface between the inner member 50 and the liner 52.
In manufacturing the bearing unit shown in FIG. 8, as shown in FIG. 9, an inner member 50 having a liner 52 of a low friction high polymer material placed on its outer peripheral surface is located inside a mold cavity 53, which corresponds in shape to the outer contour of the outer member 51, as a molding core. Then, a low melting point alloy in a molten state is poured into the cavity to define the outer member 51 by forging. The alloy thus poured is then allowed to cool to solidify so that the liner 52 becomes fixedly attached to the outer member 51 thus formed. Described more in detail in this respect, when the alloy poured into the cavity is cooled to solidify, the alloy in the cavity becomes somewhat shrunk so that the liner 52 placed on the inner member 50 comes to be compressed, thereby preventing the inner member 50 from moving freely relative to the outer member 51. Thus, after forging, as shown in FIG. 10, an external force is applied to have the outer member 51 compressed in the axial direction so as to have the outer member 51 slightly deformed to thereby provide a predetermined small gap between the inner member 50 and the liner 52. Because of the provision of such a gap, the inner member 50 may move pivotally relative to the outer member 51 and thus the liner 51 fixedly attached to the outer member 51.
However, the prior art spherical surface sliding bearing unit as described above has the following disadvantages.
In the first place, at the time of forging, the liner 52 is partly melted to be fixedly attached to the inner surface of the outer ring 51. However, since the outer member 51 is formed from a metal and the liner 52 is comprised of a resin, the coupling between these elements is inherently weak so that the liner 52 tends to be separated away from the outer ring 51 during use, particularly when a shear is applied between them. As a result, the liner 52 gradually becomes shifted in position so that the outer ring 51 may come into direct contact with the inner ring 50, which could provide an increased sliding resistance and a biased wear to the sliding surface of at least one of the inner and outer members 50 and 51 which, in turn, could produce an undesired play in a sliding motion.
Second, since the liner 52 is comprised only of a resin material, it is inherently low in the resistance against compression, so that when a relatively large compression force is applied thereto by the inner ring 50, a damage may result in the liner 52. Thus, there is a limit in the ability to bear a load by the liner 52. In addition, since a fluorine family resin material used in forming the liner 52 is relatively low in thermal conductivity, the liner 52 cannot be instrumental in dissipating the heat produced due to friction between the inner member 50 and the liner 52, and, thus, the sliding surfaces can become overheated and in the worst case stuck due to burning.
Third, while forming the outer member 51 by forging, it is important that a measure be taken to prevent any molten metal from escaping and coming into contact with the inner member 50. This is because if any of the molten metal sneak into an interface between the inner ring 50 and the liner 52, when the molten metal becomes hardened, an undesired metal comes to be present between the inner member 50 and the liner 52 and can cause an undesired wear to the sliding surface of the inner member 50. In addition, the liner 52 may be damaged to impair the performance of the bearing unit.
Fourth, as shown in FIG. 10, in the prior art process, after forging the outer member 51, an external force is applied to the outer member 51 in the axial direction after forging so as to provide a small gap at the interface between the inner member 50 and the liner 52. According this prior scheme, the outer member 51 becomes bent in the shape of a bow in cross section so that the gap thus formed is the largest at the center of the outer ring 51 and the smallest at each edge 51a of the inner member 50. As a result, the sliding resistance presented by the sliding surface of the inner member 50 becomes non-uniform, and it can impair a smooth movement of the inner member 50. Besides, since the gap is not uniform, the liner 52 may come to contact with the inner ring 50 only at its edges 51a when loaded and the contact between the inner member 50 and the liner 52 becomes local so that a biased wear could take place at these edges 51a. In order to cope with this situation, the gap must be designed to be relatively large, which is disadvantageous because of incapability to withstand a large impact load and a relatively large play resulting therefrom.